Is it possible to call multiple phones (12 phones at once) deskphone and cell phones in engagment designer or is there a limit of how many calls can go out simultaneously?

There is no limit in Engagement Designer, I commonly call 3-4 numbers during one workflow. I crafted the attached quick workflow that takes 6 numbers as input along with a message, then calls all 6 numbers at once, plays the message then drops the call after 5 seconds. This is a brute-force approach, it will scale but can/should be done more elegantly using sub-processes.